Abstract

BACKGROUND:

Externally irrigated radiofrequency (RF) electrodes have been widely used to thermally ablate tumors in surface tissue and to thermally coagulate the transection plane during a surgical resection. As far as we know, no mathematical model has yet been developed to study the electrical and thermal performance of these electrodes, especially the role of the saline layer that forms around the electrode.

METHODS:

Numerical models of a TissueLink device model DS3.0 (Salient Surgical Technologies, Portsmouth, NH, USA) were developed. Irrigation was modeled including a saline layer and a heat convection term in the governing equation. Ex vivo experiments based on fragments of bovine hepatic tissue were conducted to obtain information which was used in building the numerical model. We compared the 60°C isotherm of the computer results with the whitening contour in the heated samples.

RESULTS:

Computer and experimental results were in fine agreement in terms of lesion depth (2.4 mm in the simulations and 2.4 ± 0.6 mm in the experiments). In contrast, the lesion width was greater in the simulation (9.6 mm vs. 7.8 ± 1.8 mm). The computer simulations allowed us to explain the role of the saline layer in creating the thermal lesion. Impedance gradually decreased as heating proceeded. The saline was not observed to boil. In the proximity of the electrode (around 1 mm) the thermal lesion was mainly created by the RF power in this zone, while at a further distance the thermal lesion was created by the hot saline on the tissue surface by simple thermal conduction. Including the heat convection term associated with the saline velocity in the governing equation was crucial to verifying that the saline layer had not reached boiling temperature.

CONCLUSIONS:

The model reproduced thermal performance during heating in terms of lesion depth, and provided an explanation for: 1) the relationship between impedance, electrode insertion depth, and saline layer, and 2) the process of creating thermal lesions in the tissue with this type of electrode.

Physical situation and theoretical model. A: Physical situation with the electrode placed perpendicular to the tissue surface before saline infusion. B: Detail of the saline layer around the electrode. C: Geometry of the theoretical model (out of scale, dimensions in mm).

Modeling of the geometry of saline layer around electrode. Estimation of the geometry of the saline layer (grey zone) around the electrode surface (out of scale). The height (h) of the layer depends on the distance to the electrode surface (r). The electrode is assumed to be inserted into the tissue to depth D. The saline velocity distribution in the layer was calculated by solving the Navier–Stokes equations. A boundary condition of constant velocity was set on a specific zone of the electrode surface (red line).

Impedance progress throughout the experiments. Three phases can be observed: A) Electrode placed on tissue surface without saline infusion. B) Saline is infused and impedance drops drastically; the saline initially accumulates around the electrode, forming a pool (i.e. the height h of the saline layer increases considerably) possibly due to the depression caused by the pressure of the electrode on the tissue. Some seconds after continuous infusion sets the retained saline in motion the pool disappears, the height h decreases slightly and consequently impedance rises (arrow) until reaching a steady state. C) During RF heating tissue temperature increases gradually and impedance decreases in proportion.

Thermal lesions created after RF heating. Side (A) and surface (B) view of the thermal lesions created. After RF heating each lesion was cross-sectioned and the side view was photographed. On each side view, depth (D) and width (W) was measured. As observed in the surface view, the thermal lesions were generally non symmetrical with respect to electrode location (solid line), showing a non circular surface lesion (dotted line).

Evolution of the temperature distributions during RF heating (scale in °C). Black line represents the 60°C isotherm which is used for comparison with the whitening contour observed in the heated samples in the experimental model. Note that 1) no boiling temperatures (around 100°C) are observed in the saline layer, and 2) the heating in the tissue far from the electrode is mainly caused by the heated saline flowing over it (arrowhead). In other words, the tissue temperature is higher than the saline temperature beneath the electrode, whereas the saline temperature is higher than the tissue temperature at distant points (white line represents the 50°C isotherm).

Evolution of the temperature distributions without saline motion. Evolution of the temperature distributions (scale in °C) during RF heating considering that saline velocity is zero. Note that in this case boiling temperatures (around 100°C) appear in the saline.

Evolution of the temperature distributions in the case of in vivo situation. Evolution of the temperature distributions (scale in °C) during RF heating considering an in vivo setting, i.e. including the blood perfusion term, and with a initial temperature of 37°C.